84 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 



(1864, p. 47, pi. 15, f. 12). But here the posterior end is blunt, and the 

 sculpture consists only of concentric ribs, and above the posterior ridge 

 there are no ribs. 



I cannot find any other species that might be compared with this one ; 

 there are numerous species in Tertiary deposits which resemble this one 

 in external shape, but the character of the sculpture is here very peculiar, 

 especially the coexistence of concentric and radial sculpture, and the 

 presence of a larger number of concentric ribs on the escutcheon. 



25. LEDA ERRAZURIZI (Philippi). 

 . . PL xxvi, Fig. 3 "' 6 . 



1887 Micula err., Philippi, Tert. Quart. Verst. Chiles, p. 196, pi. 41, f. 1 1. 

 1900 Leda err. Ortmann, in: Amer. Journ. Sci., v. 10, p. 378. 



Shell elongate-ovate, apex situated almost in the middle of the dorsal 

 margin. Anterior end rounded, posterior acute, but hardly produced 

 (about as long as anterior). Posterior dorsal margin straight. Ventral 

 margin arcuate. A ridge runs from the apex to the posterior end ; just 

 below this ridge is a broad and sharply defined depression. Surface of 

 shell with fine concentric ribs, the ribs terminating, near the apex, ab- 

 ruptly on the ridge, leaving the escutheon smooth (except for growth- 

 lines ; toward the ventral margin, the concentric ribs disappear when 

 reaching the depression below the ridge, so that this depression is crossed 

 only by fine growth-lines. Anterior hinge teeth ca. 21, posterior ca. 16. 



Length, 13 mm; height, 7 mm. 



Remarks: The specimen from Santa Cruz is the largest, and shows all 

 the characters given above. Specimens from Lake Pueyrredon are 

 smaller, and are mostly casts, with only fragments of the shell remaining. 

 One individual however, shows distinctly the characters of the sculpture : 

 the disappearance of the concentric ribs on the depression below the 

 escutcheon, and therefore there is no doubt that they belong to this 

 species. The outer cast from Arroyo Gio shows distinctly the sculpture 

 of this species. The cast from Sierra Oveja is doubtful, as it does not 

 exhibit any surface characters ; but the outline agrees well with this 

 species. 



In Philippi's description of this species the surface sculpture is described 

 as " concentrice et tenuiter striata; striis ante marginem dorsalem abrupte 



